Re: [Xen-devel] Locking on vm-event operations (monitor)

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 7/22/2016 12:51 PM, Andrew Cooper wrote:
> On 22/07/16 10:27, Corneliu ZUZU wrote:
>
> > Hi,
> >
> > I've been inspecting vm-event code parts to try and understand when 
> > and why domain pausing/locking is done. If I understood correctly, 
> > domain pausing is done solely to force all the vCPUs of that domain 
> > to see a configuration update and act upon it (e.g. in the case of a 
> > XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG that enables CR3 monitoring, 
> > domain pausing/unpausing ensures immediate enabling of CR3 
> > load-exiting for all vCPUs), not to synchronize concurrent operations 
> > (lock-behavior).
> Correct.  Without the vcpu/domain pause, a vcpu could be midway 
> through a vmexit path with the monitor configuration changing under 
> its feet.  OTOH, it could be running in guest context, and only 
> receive the update one scheduling timeslice later.
> > As for locking, I see that for example vm_event_init_domain(), 
> > vm_event_cleanup_domain() and monitor_domctl() are all protected by 
> > the domctl-lock, but I don't think that's enough.
> > Here are a few code-paths that led me to believe that:
> > * do_hvm_op()->monitor_guest_request() reads 
> > d.monitor.guest_request_* resources, but it doesn't seem to take the 
> > domctl lock, so it seems possible for that to happen _while_ those 
> > resources are initialized/cleaned-up
> > * monitor_guest_request() also calls 
> > monitor_traps()->...->vm_event_wait_slot()->...->vm_event_grab_slot() 
> > which attempts a vm_event_ring_lock(ved), which could also be called 
> > _while_ that's initialized (vm_event_enable()) or cleaned-up 
> > (vm_event_disable())
> > * hvm_monitor_cr() - e.g. on the code-path 
> > vmx_vmexit_handler(EXIT_REASON_CR_ACCESS)->vmx_cr_access(VMX_CONTROL_REG_ACCESS_TYPE_MOV_TO_CR)->hvm_mov_to_cr()->hvm_set_cr0()->hvm_monitor_crX() 
> > there doesn't seem to be taken into account the possibility of a 
> > concurrent monitor_init_domain()/monitor_cleanup_domain()
> > Am I missing something with these conclusions?
> > As a resolution for this, I've been thinking of adding a 
> > 'subsys_lock' field in the vm_event_domain structure, either spinlock 
> > or rw-lock, which would be initialised/uninitialised when d.vm_event 
> > is allocated/freed (domain_create()/complete_domain_destroy()).
> I don't think you are missing anything.  It seems like a monitor lock 
> is needed.
> FWIW, the domctl lock is only used at the moment because it was easy, 
> and worked when everything was a domctl.  Being a global spinlock, it 
> is a severe bottlekneck for concurrent domain management, which I need 
> to find some time to break apart, so the less reliance on it the 
> better from my point of view.
> ~Andrew
I was thinking of not touching the way the domctl lock is acquired in 
these cases at all, but rather leave that as it is and introduce the 
rw-lock separately. Are you suggesting I should also change do_domctl to 
skip domctl_lock_acquire()-ing if op->cmd is XEN_DOMCTL_vm_event_op ? 
That would certainly be a first step towards amending that bottleneck 
you mentioned, but it would break consistency of locking-behavior for 
domctls. I think it would also add significant complexity to what I'll 
be doing so I hope it wouldn't be a problem if we leave that for a 
future patch.
Corneliu.

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